On your Facebook feed or timeline 'Right Click' on any video then 'Copy Video URL at current time'. See screenshot below.
Go to TubeLuv.com. Then paste the URL and Click 'Download', like the screenshot below
Click on "Download normal quality video" or "Download HD video" (if available) to start downloading the video.
Note 1:You can find your downloaded videos under the 'Downloads' section in Firefox menu.
Note 2:If Firefox isn't available in your AppStore, you can use any other Web Browser that allows downloading (other than Chrome and Safari).